a) Field of the Invention
The present invention relates to a vari-focal viewfinder optical system for use with cameras capable of changing photographing range into different photographing range by switching regions of films on which images are to be photographed, and more specifically to a vari-focal viewfinder optical system capable of changing a magnification thereof in conjunction with the changes of the photographing ranges of cameras which comprise photographic lens systems configured separately from viewfinder optical systems and permit photographing in a panoramic mode.
b) Description of the Prior Art
In the recent years, it is generally practised, by using cameras designed for use with films having a size of 35 mm, to perform photographing in the panoramic mode for emphasizing wide impressions or obtaining horizontally elongated photographing regions on films by trimming the photographing ranges with shielding plates which are disposed in the vicinities of surfaces of the films so as to cover upper and lower portions thereof.
However, each of the conventional viewfinder optical systems for use with cameras permitting photographing in the panoramic mode uses a visual field frame for displaying a photographing range in an ordinary mode and another visual field frame for displaying a different photographing range in the panoramic mode at the same time. In other words, the conventional viewfinder optical system uses a visual field frame 10 for displaying the photographing range in the ordinary mode and another visual field frame 11 (or marks indicating a visual field frame for displaying the photographing range in the panoramic mode) engraved within the visual field frame for the ordinary photographing mode.
Since the visual field frame for the panoramic mode is overlapped with an image to be photographed in the ordinary mode, the visual field frame for the panoramic mode hinders observation of the image to be photographed in the ordinary mode and portions outside the photographing range are visible in the panoramic mode, whereby the conventional viewfinder optical system has a defect that the optical system makes it rather difficult to correctly guess impressions of actually photographed images. In order to correct these defects of the conventional viewfinder optical systems, it is possible to adopt a method by which only the photographing range for the panoramic mode can be observable by varying the visual field frame for the ordinary mode so as to have a shortened vertical size as illustrated in FIG. 2A and FIG. 2B. However, this method narrows an area to be occupied by the visual field frame within the visual field frame for the ordinary mode of the viewfinder optical system, thereby giving an impression to photographers that the visual field for the panoramic mode is too narrow as compared with that for-the ordinary mode.
As a conventional example of viewfinder optical systems which are free from the strange impression described above, there is known a viewfinder optical system disclosed by Japanese Patent Kokai Publication No. Hei 1-129,225. This viewfinder optical system is configured so as to permit enhancing a magnification thereof and enlarging a portion of a visual field thereof by changing a focal length of an eyepiece lens component of the viewfinder optical system, thereby facilitating to obtain the trimming effect within the visual field of the viewfinder optical system.
However, both the examples of the conventional viewfinder optical systems allow aberrations before changes of magnifications of eyepiece lens components of the viewfinder optical system to be remarkably varied from those after the changes of the magnifications, and exhibit remarkably degraded optical performance after the changes of the magnifications. Further, both the examples of the conventional viewfinder optical systems adopt the eyepiece lens component which are configured as vari-focal lens components and therefore have another defect that the viewfinder optical systems are complicated in mechanical structures thereof.
Furthermore, as a still another example of conventional viewfinder optical systems which permit enhancing observing magnifications by enhancing magnifications of viewfinder optical systems with a converter lens component interposed before eyepiece lens components of secondary image formation type of viewfinder optical systems so as to change focal lengths of the eyepiece lens components, there is known a viewfinder optical system proposed by Japanese Patent Kokai Publication No. Hei 2-158,707. However, this conventional example is of the secondary image formation type (a viewfinder optical system of a type configured so as to perform reimaging of an image formed by an objective lens system and allow this image to be observed through an eyepiece lens component) which has a composition hardly permitting shortening a total length of the viewfinder optical system and is undesirable for use in compact cameras.
In addition, a viewfinder optical system proposed by the inventor as Japanese Patent Application No. Hei 4-84,136 is of a primary image formation type (a viewfinder optical system of a type configured so as to allow observation of an image formed by an objective lens system) which permits enhancing an observing magnification by changing a focal length of an eyepiece lens component with a lens component which can be disposed and removed into and out of an optical path of the eyepiece lens component. However, this viewfinder optical system requires a lens component which is to be moved along an optical axis in the eyepiece lens component for minimizing variations of aberrations and favorable adjustment of diopter before and after a magnification change of the eyepiece lens component, thereby being complicated in mechanical structure thereof.